Ink-jet printing is a non-impact printing process in which droplets of ink are deposited on a print medium in a particular order to form alphanumeric characters, area-fills, and other patterns thereon. Low cost and high quality of the hardcopy output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers.
An ink-jet image is formed when a precise pattern of dots is ejected from a drop-generating device, known as a "printhead", onto a printing medium. The typical ink-jet printhead has an array of precisely formed nozzles in an orifice plate attached to a thermal ink-jet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) from a supply channel (or ink feed channel) leading from one or more ink reservoirs. Each chamber has a thin film resistor, known as a "firing resistor," located opposite the nozzle. A barrier layer located between the substrate and the orifice forms the boundaries of the firing chamber and provides fluidic isolation from neighboring firing chambers. The printhead is mounted on and protected by an outer packaging referred to as a print cartridge.
When the resistor is heated, a thin layer of ink above the resistor is vaporized to create a drive bubble. This forces an ink droplet out through the nozzle. After the droplet leaves and the bubble collapses, capillary force draws ink from the ink feed channel to refill the nozzle.
The ink feed channel is carefully designed to provide optimal fluidic resistance. Optimal resistance guarantees that the meniscus in the nozzle returns to its equilibrium position in the minimum amount of time after firing of a drop of ink. This optimal fluidic resistance balances the need for quick refill against the need for well-behaved (well-damped) refill dynamics. The fluidic resistance is necessary to provide sufficient damping of ink movement in the nozzle during the refill portion of a drop ejection cycle. The properties of the ink greatly affect the damping requirements of the printhead. For example, less viscous inks reduce damping while more viscous inks increase damping.
In an under damped system, fluid rushes back into the ink-jet nozzle area so rapidly that it overfills the nozzle, creating a bulging meniscus. The meniscus then oscillates about its equilibrium position for several cycles before settling down. Extra fluid in the bulging meniscus adds to the volume of the emerging drop, while a retracted meniscus reduces the volume of the drop. The bulging meniscus in an underdamped pen can also lead to puddling of ink in the orifice plate surrounding the orifice bores. These ink puddles can interfere with proper drop ejection causing nozzle trajectory errors or even altogether blocking drop ejection.
In over damped pens the refill dynamics are too slow to keep up with the firing pulses sent by the printer. The result is that the pen is consistently firing on a retracted meniscus. Firing faster than the firing chamber can refill itself can also cause ingestion of air into the printhead, which results in erratic drop ejection.
For a given ink, the damping of the system can be increased by increasing the resistance of the ink refill channel. One way to do this is to lengthen the channel. An alternative way of increasing the resistance of the channel is by decreasing the channel cross section. The refill frequency which is dependent on damping is critical in designing high throughput ink-jet printing systems.
Ink-jet printheads having multiple chambers, where each chamber is dedicated to a given ink formulation are known in the art, such as that described in U.S. patent application Ser. No. 08/500796, now U.S. Pat. No. 5,734,344 by Weber et. al., entitled "Particle Tolerant InkJet Printhead Architecture." These multi-chamber printheads contain many firing chambers that are typically arranged in a group around an ink supply plenum for efficient and high quality printing. Additional groups of firing chambers may be located in the printhead to allow for individual ink colors to be printed from each group. In these multiple chamber printheads ink properties may vary for each ink color, and thus vary from chamber to chamber. Differences in key ink properties (e.g., surface tension, viscosity) may arise from attributes such as differences among the dyes, the ink vehicle, or other ink components and their concentrations. These differences in the inks lead directly to chamber-to-chamber differences in refill characteristics, as measured by F.sub.2ss. F.sub.2ss is the frequency above which the weight of ejected droplets is always less than the steady-state drop weight. At frequencies above F.sub.2ss the pen is always firing with a retracted meniscus. In existing multi-chamber printheads, the same barrier design is used for all chambers even though each chamber uses a different ink. The use of the same barrier design may lead to one chamber being over damped while another is under damped.
Thus, there exists a need for a multi-chamber printhead barrier design that is compensated for differences in ink properties.